Most of our work during the coming year will be concerned with the afferent connections of specific cells in the superior colliculus of the grey squirrel. First, the cells of origin for each efferent pathway of the superior colliculus are being identified by injecting horseradish peroxidase (HRP) in each tectal target, and filling the cells of origin by means of retrograde axonal transport. Efforts are being made to increase the uptake of horseradish peroxidase and the subsequent retrograde filling of the cell soma and dendrites by injecting the HRP with various other substances such as DMS and saponin. We can now get sufficient retrograde filling to characterize the cells of origin in terms of the morphology and laminar distribution of their dendritic fields. We have developed our electron microscopic autoradiographic techniques and plan to use these with anterograde axonal transport of tritiated amino acids to describe the afferent connections of single neurons with known efferent projections.